drivers: timer: nrf_grtc_timer: Optimize z_nrf_grtc_timer_get_ticks

Converting absolute system ticks to GRTC ticks is simple and algorithm
can be simplified. Legacy algorithm was copied from nrf_rtc_timer which
back then was working on 32 bits only.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>